Transistors may be formed by processes wherein their semiconducting layer, and in many cases, other layers is deposited from solution. The resulting transistors are called thin-film transistors. When an organic semiconductor is used in the semiconducting layer, the device is often described as an organic thin film transistor (OTFT).
Various arrangements for OTFTs are known. One device, an insulated gate-field effect transistor, comprises source and drain electrodes with a semiconducting layer disposed therebetween in a channel region, a gate electrode disposed over the semiconducting layer and a layer of insulating material disposed between the gate electrode and the semiconductor in the channel region.
The conductivity of the channel can be altered by the application of a voltage at the gate. In this way the transistor can be switched on and off using an applied gate voltage. The drain current that is achievable for a given voltage is dependent on the mobility of the charge carriers in the organic semiconductor in the active region of the transistor, namely the channel region between the source and drain electrodes. Thus in order to achieve high drain currents with low operational voltages, organic thin film transistors must have an organic semiconducting layer which has highly mobile charge carriers in the channel region.
High mobility OTFTs containing small molecule organic semiconductors have been reported and the high mobility has been attributed, at least in part, to the highly crystalline nature of the semiconductor. Particularly high mobilities have been reported in single crystal OTFTs wherein the organic semiconductor is deposited by thermal evaporation (see, for example, Podzorov et al, Appl. Phys. Lett., 2003, 83(17), 3504-3506).
Unfortunately, however, it can be difficult to obtain repeatable results from solution processed films of small molecule semiconductors and this is believed to be due to their poor film forming properties. Issues with material reticulation from and adhesion to substrates, film roughness and film thickness variations can limit the performance of small molecule semiconductors in OTFTs. Film roughness can be a further problem for top gate organic thin film transistors as the accumulation layer is formed at the uppermost surface of the semiconducting layer.
To overcome this problem, the use of blends of small molecule semiconductor and polymers, especially polymeric semiconductors, has been developed. The motivation for using such blends is primarily to overcome the poor film forming properties of the small molecule semiconductors. Blends exhibit superior film forming properties due to the film forming properties of the polymer. Numerous examples of small molecule semiconductor and polymeric semiconductor blends can be found in the literature.
Blends of small molecule semiconductor and polymeric semiconductor may be solution processed, e.g. by spin coating or ink jet printing, to form a semiconducting layer. Generally the process involves dissolving the semiconductors in a solvent, spin coating or ink jet printing the solution onto a substrate and then drying the resulting wet film. During the drying step, the solvent evaporates to yield the semiconducting layer comprising a matrix of polymer semiconductor comprising crystals of small molecule semiconductor.
Generally an aromatic or substituted aromatic solvent is used to dissolve the semiconductors. Most commonly o-xylene is used. The driving factor in the selection of this solvent is the fact that it dissolves both small molecule and polymeric semiconductors and it evaporates quickly to form the semiconducting layer.
GB2482974, however, discloses a method of making OTFTs wherein a solvent selected from C1-4 alkoxybenzene and C1-4 alkyl substituted C1-4 alkoxybenzenes is used instead of o-xylene. The specific solvents exemplified are o-xylene, tetralin, 3,4-dimethylanisole, anisole and mesitylene. GB2482974 teaches that the mobilities achieved by the use of anisole compared to the other solvents is greater and as a consequence the device performance is improved. The improved device performance is realised at short channel lengths due to a reduction in contact resistance when such solvents are used.
Most of the prior art relating to small molecule and polymeric semiconductor blends have focussed on selection of certain semiconductors and their ratios in the blend in order to optimise the field effect mobility.